1. Field of the Invention
This invention relates to a flexible substrate strip for use in forming a plurality of substrate-based semiconductor chip packages wherein the excess encapsulant can be removed without damaging the packaged electronic device after encapsulating the semiconductor chip.
2. Description of the Related Art
As the need for lighter and more complicated semiconductor devices becomes greater day by day, the semiconductor chips have become more and more complex thereby requiring more electrical connections. Therefore, the ball grid array (BGA) has been developed by the semiconductor chip packaging industry to meet these needs.
A typical BGA package generally includes a chip mounted to the upper surface of a substrate i.e. a printed circuit board. Bonding pads on the active surface of the chip are connected to electrically conductive traces formed on the upper surface of the substrate by bonding wires. The lower surface of the substrate is provided with a plurality of solder pads electrically connected to the electrically conductive traces. Each solder pad is mounted with a solder ball for making external electrical connection. A package body is formed to enclose the chip, the bond wires and a portion of the upper surface of the substrate including most of the electrically conductive traces. The package body is typically formed by a transfer molding process.
FIG. 1 is a top plan view of a conventional transfer molding equipment with flexible substrate strips attached. As shown, the molding equipment includes three transfer pots 201 for accommodating encapsulant. Each transfer pot 201 of the molding equipment has a transfer rams 204 positioned therein and is connected to four gates 208 through a mold runner 206. Each gate 208 is connected to a cavity 210. Two flexible substrate strips 230 are placed in the molding equipment in a manner that each substrate unit of the strips 230 adapted for mounting a semiconductor chip is corresponding to each cavity 210 of the molding equipment.
After the encapsulant is loaded in the transfer pots 201 and the flexible substrate strips 230 are fixed and clamped by the molding equipment, the transfer ram 204 is moved downwardly to compress the encapsulant. The molding equipment and encapsulant are pre-heated so that when the transfer ram 204 compresses the encapsulant, the liquefied encapsulant is forced through the mold runners 206 and gates 208 to fill the cavities 210 and thereby encapsulating the semiconductor chips (not shown) mounted on the flexible substrate strips 230. After the encapsulant fills the cavities 210, the transfer ram 204 stands still for a predetermined time until the encapsulant cures. Then the transfer ram 204 is withdrawn, the molding equipment is opened, and the molded products is removed from the molding equipment. Extra parts such as runners and gates are removed from the molded products, and then the molded products are cut into individual units, whereby the semiconductor chip packages are completed.
However, one shortcoming of the above process is apparent. Specifically, the encapsulant not only fills the cavities 210 but also fills the gates 208, the mold runners 206 and the transfer pots 201. Therefore, when the encapsulant is cured, the cured encapsulant not only covers the semiconductor chips, but also extends along the surface of the flexible substrate strip 230, where the gates 208 and the mold runners 206 are located, and into the pots 201. This excess cured encapsulant is often referred to as the "runner" and must be removed before the molded products are singulated. Accordingly, the gate 208 is generally made smaller in cross-sectional area than the mold runner 206 in order to assist in the "degating" process, i.e., the removal of the excess encapsulant. However, the encapsulant tends to adhere to the surface of the substrate, so the removal of the excess encapsulant is likely to twist the flexible substrate strip and causes damage to the surface thereof.
Therefore, it is desirable to provide degating regions 220 on the flexible substrate strip 230 such that the edges of mold runners 206 and gates 208 fit entirely within the degating regions 220 during encapsulation of the chips. Typically, a degating region material such as gold is formed on the degating regions wherein the adhesive force between the encapsulant and the degating region material is less than the adhesive force between the encapsulant and the substrate whereby the excess encapsulant can be removed without damaging the flexible substrate strip.
FIG. 2 is a partial side view of a packaged product 240 with to-be-removed excess encapsulant. As shown in FIG. 2, the gate 208 (for simplicity, the mold runner 206 and the transfer pot 201 are not shown in FIG. 2) is still linked with the packaged product 240. After degating, it is desirable to have a substantially regular breaking surface as shown in FIG. 3. However, there is nearly no resistance force during the removal of the excess encapsulant from the degating region, hence the breaking force is so violent as to generate the defect results as shown in FIG. 4 and FIG. 5. FIG. 4 shows a bulge 242 protruding from the outline of the packaged product. The bulge 242 will cause damage to the punch tool during the singulation process. FIG. 5 shows a concave 244 formed in the packaged product. If the damage caused by the formation of the concave 244 is severe enough, it may harm electrically conductive traces on the flexible substrate strip. Even if there is no direct damage to the substrate, the concave 244 resulting from the degating process may still weaken the seal between the molded body and the upper surface of the substrate, thereby increasing the chances of moisture penetration in the packaged product.